1. Field of the Invention
This invention relates to a process for the vapor phase nitration of aromatic compounds. More particularly, this invention relates to a process of a vapor phase nitration of aromatic compounds in the presence of a nitration promoting catalyst comprising a phosphorous-vanadium-oxygen complex.
Nitroaromatic compounds find use as solvents, explosives, dyes, perfumes, and analytical reagents, and are important as intermediates in organic synthesis. As an example, nitroaromatic compounds are convertible by reduction into primary amines, which, in turn, are valuable intermediates in the synthesis of dyes, pharmaceuticals, photographic developers, antioxidants, and gum inhibitors.
2. Description of the Prior Art
Nitroaromatic compounds are currently produced primarily via liquid phase reactions employing mixed acids. A sulfuric acid/nitric acid mixture is the most commonly employed industrial nitrating agent. Other mixed acids for nitration of aromatic compounds are acetic acid/nitric acid mixtures as described, for example, in U.S. Pat. No. 3,180,900. In U.S. Pat. No. 3,928,476, the latter type nitration is conducted over silica-alumina or alumina supports.
Vapor phase nitration of aromatic compounds is also known in the art. The vapor phase nitration of benzene and toluene at temperatures ranging from about 275.degree. C. to about 310.degree. C. is described in McKee and Wilhelm, Industrial and Engineering Chemistry, 28(6), 662-667 (1936) and U.S. Pat. No. 2,109,873. McKee and Wilhelm catalyzed their reaction with silica gel, with best results being reported by the use of 14 mesh material. Bauxite and alumina were reported to be ineffective as catalysts in the vapor phase nitration of benzene. More recently, U.S. Pat. No. 4,107,220 described the vapor phase nitration of chlorobenzene in the presence of molecular sieve catalysts having a pore size varying from about 5 A to about 10 A as a means for controlling the para-to-ortho isomer distribution of nitrochlorobenzene. A suitable temperature range was reported to be from about 190.degree. C. to about 290.degree. C.
Although these prior art processes generally provide the desired product, the choice of available catalysts is severely limited in that each of the prior art processes uses aluminosilicates (or silica gel) as catalysts. Thus, the discovery of an alternate catalyst composition suitable for use in the vapor phase nitration of this invention would be a decided advance in the art.